1. Field of the Invention
The present invention is for a sports timing device and, more particularly, for a method and apparatus for monitoring wet contact touchpads in a swim timing system that allows accurate detection of both the actuation of and the release of a switch-like wet contact touchpad while preventing corrosion of the switch contact material. More particularly, the wet contact touchpads consist of flexible metal electrode plates separated by non-conducting spacers forming a simple switch. The space between the flexible metal electrode plates is allowed to fill with water when the wet contact touchpad is placed in a pool. The large parallel flexible metal electrode plates separated by water form a large capacitor which must be charged up by the monitoring circuit before a switch closure can be sensed. The potential across the plates must not exceed the ionization voltage of water (˜1.3V) or corrosion will occur.
2. Description of the Prior Art
In prior art devices, the capacitance formed by the opposing flexible metal electrode plates of a wet contact touchpad does not present a significant problem if only closures need to be monitored because the voltage across the plates will change rapidly when they are shorted together. Accurately detecting the release is more difficult. A wet contact touchpad can be modeled as a large capacitor in series with a small resistor. A common charging method is to pull one flexible metal electrode plate of the wet contact touchpad up to a voltage less than 1.3V, which results in a series RC circuit where the voltage across the capacitor formed by the flexible metal electrode plates is an exponential function of time. The large wet contact touchpad capacitance makes the time constant of this circuit too large to allow accurate sensing of the release.
Directly monitoring the voltage across the wet contact touchpad while charging is impractical because of the series resistance in the wet contact touchpad model. The voltage across the capacitor plus the voltage drop across the resistor will always equal the full charging voltage. In addition, the capacitance and the resistance in the wet contact touchpad model are not fixed and may vary depending on wet contact touchpad age, size, and pool chemistry, which makes simple level detector circuits inadequate.